Chemical milling method and bath

ABSTRACT

CHEMICAL MILLING OF TITANIUM AND TITANIUM ALLOYS BY IMMERSING THE SAME IN A BATH COMPRISING 2% TO 7% BY VOLUME HYDROFLUORIC ACID, 1% TO 10% BY VOLUME HYDROCHLORIC ACID, WITH REAMINDER WATER, FOR SUCH TIME, AND AT SUCH TEMPERATURE AS TO REMOVE A DESIRED QUANTITY OF METAL AND YIELD A DULL SATINLIKE FINISH. FOR A BEST SURFACE FINISH, IMMERSION IN THE HYDROFLUORIC ACID-HYDROCHLORIC ACID BATH IS FOLLOWED BY IMMERSION IN A BATH COMPRISING 5% TO 20% BY VOLUME NITRIC ACID AND 2% TO 5% HYDROFLUORIC ACID, WITH REMAINDER WATER, THIS AT SUCH TEMPERATURE AND FOR SUCH TIME AS TO CLEANSE THE SURFACE WITHOUT, HOWEVER, POLISHING THE SAME.

United States Patent Int. Cl. C231 1/00 US. Cl. 156-18 6 Claims ABSTRACT OF THE DISCLOSURE Chemical milling of titanium and titanium alloys by immersing the same in-a bath comprising 2% to 7% by volume hydrofluoric acid, 1% to 10% by volume hydrochloric acid, with remainder water, for such time, and at such temperature as to remove a desired quantity of metal and yield a dull satinlike finish. For a best surface finish, immersion in the hydrofluoric acid-hydrochloric acid bath is followed by immersion in a bath comprising to 20% by volume nitric acid and 2% to 5% hydrofluoric acid, with remainder water, this at such temperature and for such time as to cleanse the surface without, however, polishing the same.

As a matter ofintroduction, our invention generally is concerned with titanium and titanium alloy products.

One of the objects of our invention is the provision of a comparatively inexpensive method for chemically removing metal from the surface of titanium and the titanium alloys.

Another object is the provision of a method for removing metal from the surface of titanium and titanium alloys in the form of bar and billet, sheet, strip and plate, wire, tubes, special extruded shapes, and the like, to eliminate seams, fissures or other surface discontinuities or to so modify the same as to permit correction by subsequent mechanical processing.

Another object is the provision of a bath of removing metal from the surface of titanium and titanium alloys, all with maximum life of bath, and minimum cost.

Other objects of our invention in part will readily appear from the description which follows and in part will be more particularly pointed to.

The invention, accordingly, consists in the combination of the ingredients forming the bath, in the several operational steps in which the bath is employed, and in the relation of each of these steps to one or more of the others, all as more fully described herein, the scope of the application of which is set out in the claims at the end of this specification.

BACKGROUND OF THE INVENTION In order to better understand certain features of our invention, it may be noted at this point that titanium and the titanium alloys are coming into rather widespread use where a savings in weight justifies an increase in cost over other metals. The unalloyed titanium is suited to many applications where ductility and formability are required, along with resistance to corrosion and heat, particularly where light weight is critically important. In the aircraft industries, the unalloyed titanium is particularly suited to ducting, shrouds, stifieners, firewalls and fasteners. The metal also is suited to marine applications. And it commonly is used as weld rod material for the welding of titanium and its alloys.

Where greater strength is required, the art may turn to the alpha titanium alloys. One typically is identified as containing about 5% aluminum, 2.5% tin, and remainder titanium. This alloy is strong, ductible and enjoysgood creep-resistance up to about 900 It is'readily welded;

and is suited to numerous aircraft and cryogenic applications. A further alloy contains about 8% aluminum, 1% molybdenum, 1% vanadium, and remainder titanium, with mechanical properties slightly improved over the 5% aluminum-2.5% tin alloy. Another contains about 7% aluminum, 2% columbium, 1% tantalum, and remainder titanium, this enjoying good high-temperature strength.

It is perhaps the alpha-beta titanium alloys which are most favored in the art, for they respond to heat-treatment. One of the more popular alloys contains about 6% aluminum, 4% vanadium, and remainder titanium, Along with good corrosion-resistance, it is characterized by elevatedtemperature strength and stability, as well as good machinability. It is made available in the form of bars, sheet, strip, wire, extruded shapes, and tubing. It well lends itself to the production of a variety of forgings. A further alpha-beta alloy contains about 6% aluminum, 2% tin, 4% zirconium, 2% molybdenum, and remainder titanium. This alloy when hardened by aging treatment enjoys even higher tensile strength. It is suited to applications involving heavy stresses at high temperatures for long periods of time, with good strength, toughness and stability at temperatures up to about 900 F. Another alloy, this with particularly good welding characteristics and fabricability, with somewhat better strength, is a titanium alloy containing about 6% aluminum, 6% vanadium, and 2% tin.

The titanium alloy having a best combination of mechanical properties, which through age-hardening develops a tensile strength in excess of 210,000 p.s.i., is the beta alloy containing about 13% vanadium, 11% chromium, 3 aluminum, and remainder titanium. This alloy is weldable and is available in the form of bars, wire, sheet and strip. For cold-heading applications the Beta III alloy may be employed, this containing about 11.5% molybdenum, 6% zirconium, 4.5% tin, and remainder titanium.

Although titanium and titanium alloys enjoy a combination of highly desirable characteristics, including a high ratio of strength-to-weight, weldability, formability, corrosion-resistance and heat-resistance, they are inclined to oxidize at hot-working temperatures. Moreover, they are extremely sensitive to hydrogen embrittlement.

While numerous processes have been developed to free the hot-worked titanium alloys of scale, there are but few which are concerned with the more subtle imperfections, such as minute hairlines, surface cracks and fissures. The scale-removing processes are costly, time-consuming, and usually require special and expensive handling equipment. For example, the well-known sodium hydride bath requires an operating temperature of some 700 F., with subsequent water rinses and acid dips. Another employing an oxidizing salt bath comprising sodium hydroxide and one or more of a number of sodium salts likewise requires high operating temperatures in order to maintain the molten condition. These both, while principally designed to remove scale, nevertheless remove some metal from the scale-freed surface.

Where it is a cleansing rather than a scale removal which is required, the art may resort to an acid pickling bath, for example, one comprising a major portion of nitric acid, a minor portion of hydrofluoric acid, and remainderwater. In some instances electrolytic pickling is recommended, the electrolytic bath typically consisting of a major amount of sulphuric acid, with minor amounts of nitric and hydrofluoric acids.

In some instances, scale is removed and the surface finished by way of mechanical methods, i.e., grinding, grit blasting, sanding, and the like. But these processes not only are time-consuming, but the results had are by no means uniform.

Accordingly, it is one of the objects of our invention 'to provide-a simple, direct, non-electrolytic method or process for preparing the surface of titanium and its alloys in the form of plate, sheet, strip, bar, rod, wire and special shapes for further processing, i.e., cold-rolling, cold-drawing, cold-forming, and the like, as by reliably removing surface metal from the same to a desired limited extent and rounding off the edges of cracks, seams, and the like, all with minimum contamination by hydrogen, with minimum oxidation, and without pitting of the metal.

SUMMARY OF THE INVENTION In accordance with the teachings of our invention, we find that the unalloyed titanium, as well as its various wellknown alloys, that is, the alpha alloy, the beta alloy, and the alpha-beta alloy, in common converted forms of bar, rod, wire, plate, sheet, strip, tubing and extrusions, as well as forgings, frequently contain minute surface discontinuities in the form of seams, cracks, hairlines and microfissures, which directly and adversely affect further processing. These various surface imperfections may be attributed to a combination of surface hydrogen contamination and working stresses encountered in conversion, that is, ingot to billet, to bar and plate, and to wire, sheet and strip. The hydrogen contamination very well may derive from decomposition of atmospheric water vapor during some prior processing, or from a reducing atmosphere encountered in the fuel-fired furnaces of some prior heating operation. The surface imperfections also may come from minute mechanical defects in the surface of the rolling equipment.

We find that by removing from the surface of the various titanium and titanium alloy converted and semiconverted products some 0.00 from the diameter or thickness of the same (0.0025" from a single surface), most of the imperfections are removed. For a best combination of results, however, We prefer to remove from the surface as much as 0.010" from the thickness or diameter, or even as much as 0.015". We find that with the removal of surface metal, virtually all of the imperfections are either eliminated or so minimized that with further processing, as by cold-rolling, cold-drawing, or the like, a uniform, high-quality surface is had.

And in accordance with the teachings of our invention, the required amount of surface metal is removed by way of what we call a chem-milling process, employing a combination of hydrofluoric and hydrochloric acids in aqueous solution. In general, the hydrofluoric acid-content ranges from some 2% to 7% by volume, with the hydrochloric acid content ranging from about 1% to 10% especially about 3% to 10%, or for best results from about 3% to 7%, or even 5% to 10%. The remainder, of course, is water.

Actually, in our chem-milling bath the hydrofluoric acid content is critical. We find that where the hydrofluoric acid content is as high as 1 there is had undesired hydrogen contamination of the surface of the metal. And where the hydrofluoric acid content is less than 2%, the bath is ineffective. For a best combination of effective metal removal and yet an assured freedom from objec-v tionable hydrogen contamination we employ hydrofluoric acid in the amount of about 2% to 7% by volume, and more especially about by volume. With the hydrofluoric acid content of about 5% the hydrochloric acid content ranges between about 3% and 1 0%, a best com, bination of results being had with the hydrochloric acid in the amount of about 5% to 7 v The bath is maintained at a temperature of some 65 to 140 F.; for best results the bath temperature is on the order of some 130 to 140 F. With the bath temperature lower than 65 F., little action is bad. And where the temperature exceeds 140 F., the action is too aggressive, and uneven and irregular attack results, giving a non-uniform surface. Moreover, at the higher temperatures there is a loss of the hydrofluoric acid content.

The time of treatment ranges from about 5 minutes to about 25 minutes, depending upon bath temperature; as

4, a result of an exothermic reactionbetween metal and bath constituents the temperature of the bath rises with each batch of metal as the practice progresses. Treatment for some 10 or minutes at the 130 to 140 F. temperature usually gives excellent results, this giving metal removal of about 0.010" off the thickness or' oif the diameter of the metal being treated (about 0.005" off the single surface). Treatment at the higher temperatures for the shorter periods of time is particularly desirable in that it minimizes the opportunity for hydrogen pick-up, with consequent deteriorating effect of surface embrittlement. The hydrogen pick-up in our view'should not exceed about 5 or 10 parts per million, this taking cognizance of the hydrogen originally present giving a total hydrogen content at the surface substantially less than 80 p.p.m., and for best results not exceeding about 50 p.p.m. V

Following treatment, the metal is washed with water under pressure and set aside for drying and subsequent processing or forming operations. The surface is characterized by a dull, satinlike appearance. This is particularly important for hot-rolled Wire destined for colddrawing, Where leading, soaping or other surface lubricating operation commonly is required.

In certain grades of titanium alloy we observe a thin smut on the surface of the product as it comes from the chemmilling bath. Particularly is this true of the alloys containing a small amount of tin; actually, We perceive some discoloration on virtually all of the titanium alloys, except perhaps that containing 6% aluminum and 4% vanadium. For a desired combination of metal removal and assured cleanliness without loss of satinlike finish we I further treat the metal by quick dip in an aqueous soluranges from some minute to one or more minutes.

Usually we find that a dip of about /2 minute gives the desired result. In this operation, in the nature of a cleansing, an excessive time must be avoided, for with time of treatment exceeding a minute or two, the metal loses its desired dull satinlike appearance and acquires an, undesired polish, mirrorlike in quality.

Although in the practice of our process no treatment of the metal is required prior to the chem-milling operation, unless, of course, there is a matter of scale removal,

this by conventional well-known method, there are instances where pretreatment in the nature of a flash pickling operation in sulphuric acid is beneficial. A quick dip in a 20% sulphuric acid solution has an advantage of removing grease and the like accidentally applied to the metal in handling the same. In general, however, the.

sulphuric acid flash pickle is not employed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As particularly illustrative of the practice of our invention, hot-rolled titanium alloy containing about 6% aluminum, 4% vanadium, and remainder titanium, in the form of rod, is immersed in a hydrofluoric acid-hydrochloric acid solution forabout 10 minutes, with the solution maintained at room temperature, that is, about 70 to F. The hydrofluoric acid content of the solution amounts to about 5% by volume and the hydrochloric acid content also about 5% by volume. The remainder, of course, is water. Examination of the metal following treatment reveals substantial freedom from seam, fissure or other surface discontinuity. The amount of metal removed is about 0.005" off the diameter.

At higher temperatures, that is, a temperature of some to F., more aggressive attack is had with treatment for 10 minutes. The amount of metal removal at this temperature is on the order of 0.010" off the diameter. Such treatment, although increasing the cost of operation through reducing the yield of metal and more quickly consuming the active ingredients of the bath, assures virtually complete elimination of seams, microfissures and other surface discontinuities.

The rate of metal removal decreases slowly with continued use of the bath. We find that when the metal content of the bath reaches approximately 2%, the rate of attack is only about 0.002 off the diameter in a 10- minute period. As an economy measure, then we spike the bath as by adding about one-half of the original amount of hydrofluoric and hydrochloric acids. The life of the bath thus is eflectively prolonged.

As particularly illustrative of the practice of our invention in large-scale production, there was prepared a LOGO-gallon aqueous solution containing hydrofluoric acid and 5% hydrochloric acid. The solution was brought up to a temperature of some 65 to 70 F. by introducing live steam. Two coils of round rod of the titanium alloy containing 6% aluminum and 4% vanadium were immersed for a period of 6 /2 minutes, then withdrawn from the bath and measured to ascertain the loss in diameter. The resulting loss amounted to about 0.005". During treatment, the temperature of the bath rose a bit as a result of the exothermic reaction between bath and metal. In further illustration, like coils immersed in the bath for 15 minutes suffered a loss amounting to 0.007" off the diameter. All coils were found to have a dull satin- ]ike appearance with no evidence of pitting and no evidence of polish. The surfaces were seen to be smooth and virtually free of scam or other discontinuity. The hydrogen content of the metal at the surface amounted to some 35 to 55 ppm.

Hydrogen analysis of a number of coils shows that the chem-milling operation results in negligible hydrogen contamination. Actual test reveals that with an initial surface hydrogen content on the order of some 34 to 49 ppm. prior to treatment, the hydrogen content subsequent to the chem-milling operation amounts only to some 37 to 54 p.p.m., a pick-up on the order of 5 ppm.

As a further example of the practice of our invention, hot-rolled unalloyed titanium in the form of a round mill product and found to have some minor surface discontinuity, was immersed in a hydrofluoric acidhydrochloric acid solution for about minutes, the solution being at a temperature of some 90 to 100 F. The hydrofluoric acid content of the solution amounted to about 5% by volume and the hydrochloric acid content also 5% by volume. The remainder, of course, was water. Examination of the metal following treatment revealed substantial freedom from seam, fissure or other surface discontinuity. The amount of metal removed was on the order of some 0.012 011? the diameter, that is, 0.006" off one surface.

Where desired, a hot-rolled titanium product prior to treatment in the hydrofluoric acid-hydrochloric acid bath first may be subjected to treatment in a molten salt bath, this principally comprising sodium hydroxide with additions of oxidizing salts. The molten salt bath treatment is applied only where the nature of the surface demands it, that is, where an objectionable oxide film is present as a result of some prior mill treatment.

The Beta 111 titanium alloy, when treated in the 5% hydrofluoric acid-5% hydrochloric acid solution for 10 minutes at 100 F, is effectively milled, as in the case of the unalloyed titanium and the 6% aluminum-4% vanadium-titanium alloy. Here some 0.010" is taken off the diameter, this giving an excellent surface, free of crack, seam or blemish. And so, too, the alloy containing 6% aluminum, 2% tin, 4% zirconium, 2% molybdenum, and remainder titanium, when similarly treated for 10 minutes at 100 F., suffers a loss of some 0.006" off the diameter, giving a smooth surface suited to subsequent processing.

With an alloy containing 6% aluminum, 6% vanadium, 2% tin, and remainder titanium, somewhat greater length of exposure is .found desirable, especially when the alloy is in the hot-rolled condition. For only 0.003 is removed from the diameter with the 10-minute treatment at F. With prior treatment in the molten salt bath, however, the chem-milling treatment for 10 minutes at 100 F. produces a loss of metal off the diameter of 0.012".

In general, we find that all of the titanium alloys, as well as the unalloyed titanium, lend themselves to chemmilling with some 0.005, or 0.010, or even 0.015 removed from the diameter of the metal with treatment in the 5% hydrofluoric acid-5% hydrochloric acid bath at 100 F. in some 10 minutes or even 15 minutes time. A surface of dull satin finish is had in every instance. This surface is virtually free of seam, microfissure, or other like imperfection. Where in those instances that a thin smut appears on the surface of the metal, this readily is removed by a quick dip in a nitric acid-hydrofluoric acid solution, as noted above.

Thus, in conclusion, it will be seen that we provide in our invention a method or process for what we term a chem-milling of titanium and its alloys in which the various objects hereinlaefore set forth are eflectively achieved. The method and process are simple, direct and eflective. There is employed a minimum of expensive materials. There is a minimum of bath fuming. And the resulting chem-milled surface is of high quality, free of surface defect and evidencing virtually no hydrogen contamination. A dull satin finish is had which in no sense may be viewed as a polish. Following treatment, the metal then is admirably suited to cold-rolling, cold-drawing, or other processing.

Inasmuch as there are many embodiments which may be made of our invention, and inasmuch as many changes and variations may be made in the embodiments set out above, it will be understood that all matter described herein is to be interpreted as merely illustrative and not by way of limitation.

We claim:

1. The art of minimizing the effect of seams, haircracks or microfissures in titanium and titanium alloyed products without substantially contaminating the same with hydrogen, which comprises treating said products in a molten salt bath principally comprising sodium hydroxide, washing the products, then treating the same in an aqueous solution of hydrofluoric acid in amount of at least 2% but less than 10% by volume and hydrochloric acid in the amount of 1% to 10% maintained at 65 to 140 F. for some 2 to 20 minutes to remove surface metal and to give a surface of dull satin finish.

2. The art of treating titanium and titanium alloyed products to minimize surface discontinuities which comprises immersing said products in a molten salt bath principally comprising sodium hydroxide, washing the products, and then treating the same in an aqueous solution of about 2% to 7% hydrofluoric acid by volume and about 3% to 10% hydrochloric acid maintained at to F. for some 2 to '20 minutes to remove surface metal and to give a surface of dull satin finish with hydrogen content not exceeding about 80 parts per million.

3. The art of treating titanium and titanium alloyed products to minimize surface discontinuities which comprises immersing said products in a molten salt bath principally comprising sodium hydroxide with additions of oxidizing salts, washing the products, then treating the same in an aqueous solution of about 2% to 7% hydrofluoric acid by volume and about 3% to 7% hydrochloric acid and maintaining the same in such bath at a temperature of some 100 to 140 F. for sufficient time to effectively remove some 0.002" to 0.015" of metal from the diameter or thickness of the product.

4. The art of treating titanium and titanium alloyed hot-worked products which comprises removing scale from the surface of the products by immersing the same in a molten salt bath principally comprising sodium hydroxide with additions of oxidizing salts, washing the products, and immediately thereafter subjecting the same to treatment in an aqueous solution of hydrofluoric acid in the amount of at least 2% but less than 10% by volume and hydrochloric acid in the amount of 1% to 10% maintained at some 100 to 140 F. for some 2 to 20 minutes to minimize surface discontinuities and give a surface of dull satin finish.

5. The art of treating titanium and titanium alloyed products which comprises removing scale from the surface of the products by immersing the same in a molten salt bath principally comprising sodium hydroxide, wash ing the same, immersing said products in an aqueous solution containing about 2% to 7% hydrofluoric acid by volume and 5% to 7% hydrochloric acid for a period of some 2 to '10 minutes while maintaining the bath at a temperature of some 100 to 140 F. whereby surface metal is removed and discontinuities minimized, and then treating the products in a 5% to 20% nitric acid-2% to 5% hydrofluoric acid solution for some minute to 2 minutes to remove any smut or other discoloration on the surface of the metal.

6. In the art of minimizing the efiect of seams, haircracks or microfissures in titanium or titanium alloyed products the method which comprises treating said products in an aqueous solution of at least 2% but less than 10% by volume hydrofluoric acid and about 3% to 10% hydrochloric acid maintained at 65 to 140 F. for some 2 to 20 minutes to minimize discontinuities and give a surface of dull satin finish; and then treating the products in a 5% to 20% nitric acid2% to 5% hydrofluoric acid solution for some 4 minute to 2 minutes to remove any smut or other discoloration on the surface of the metal, but without polishing the surface.

References Cited UNITED STATES PATENTS 2,790,738 4/ 1957 Alexander et al. 134-2 2,981,609 4/ 1961 Acker et a1. 15'618 2,981,610 4/ 1961 Snyder et a1. 15 6-18 FOREIGN PATENTS 181,475 4/ 1966 Russia.

OTHER REFERENCES I. E. Titova et al., Dissolution of Titanium in a Hydrochloric Acid Solution, Zh. Prlkl. Khim. 41 (2) 2805 (1968), p. 56, vol. 68 (1968), Chem. Abs. cited together with translation.

Battelle Mem. Inst., Machining of Titanium Alloys, DMIC Memo. .199, Feb. 2, 1965.

James et a1. Chemical Kinetics of the ZnZHF Reaction, Jour. Phys. Chem. ('64) pp. 286-88 (1960).

Habil, Matting et al., Gluing of Titanium, Metal, pp. 6-1-6, January 1962.

Straumanis et al., Electrochemical Behavior of TiO Solid Solutions in Aqueous HF-HCl, I our. the Electrochemical Soc., March 1965 Metals Handbook, vol. 2, p. 361 (1964).

ROBERT F. BURNETT, Primary Examiner R. J. ROCHE, Assistant Examiner US. Cl. X.R.' 

